Amplifying tube and method of operating same



I Dec. 11, 1934. L 'TH MP N I 1,983,714

AMPLIFYING TUBE AND METHOD OF OPERATING SAME Filed May 8, 1930 0 0 Q 0 111 1111 O 0 15 4E W 1111 o 0 0 0 21 23 INVENTOR L /NCOL/V THOMPSON BY 7I J 5 111/451 A.

ATTORNEY Patented Dec. 11, 1934 UNITED STATES AMPLIFYING TUBE AND METHODOF OPER- ATING SAME Lincoln Thompson, Naugatuck, Conn, assignor, bymesne assignments, to Radio Corporation of America, a corporation ofDelaware Application May 8, 1930, Serial No. 450,606

3 Claims.

The invention relates to thermionic tubes, such as are utilized inelectrical amplification systems, and more especially toa tubeadaptedfor amplifying systems of the nature more particularly set forthin my U. S. Letters Patent No. 1,755,938 in which tubes of relativelyvery high amplification constant are used as power tubes operated atzero or positive grid biases. Under these conditions, substantialelectron currents flow in the grid circuit of these tubes, while in theordinary amplifying systems, negative biases are applied to the grids ofthe-usual, well-known tubes to prevent the fiow of electron currents inthe grid circuit. Tubes for use in the aforesaid patented system have,therefore, been required to allow relatively large grid-current fiow;and, to facilitate the distortionless transmission of signals withthissystem, it has been required that the relation between grid voltagesand grid currents should be as nearly linear as possible.

If a pairof matched tubes, used in the compensated arrangement of thepatented system referred to, are operated at Zero grid potential, thecombined grid circuits will function like a pure resistance if thegrid-current grid-voltage curves are linear for the tubes and there is anegligibly small grid current at zero grid.

If there is a curvature in the region of zero grid, and the remainder ofthe characteristic is linear, the combined curves of the two tubes maybe made to overlap to give a continuous straight line, usually by theapplication of a slight positive bias to each tube.

If but a single one of these tubes is used, a suitable positive bias maybe applied so the operating point is in a linear region of thisgridvoltage grid-current characteristic, thus also giving a pureresistance characteristic for signals not exceeding the linear region.

It is the object of the present invention to provide tubes with adjustedgrid-voltage gridcurrent characteristic as nearly linear as possible andadapted for the fiow of relatively large electron currents in theirgrids, which are therefore to be operated at zero or positive biases.

I have found that tubes may be constructed and operated to respond tothe requirements aforesaid, in part through the selection of the properfilament voltage and particularly through the adjustment of secondaryelectron emission from the grid. Reduction of the voltage of thefilament to as low a value as practicable for the necessarily increasedfilament current for the same filament emission, tends to eliminatecurvature in the grid-voltage grid-current characteristic near zero gridpotential. By adjustment of the secondary emission from the grid todesired amounts, the net grid currentwhich is represented by thedifference between the primary electrons entering the grid and thesecondary electrons leaving said grid-may be reduced to a degree suchthat the grid-voltage grid-current characteristic may be made verynearly linear.

The mechanism of secondary emission from the grid is as follows. Whenthe grid is negative, there is no electron-grid-current flow or flow ofprimary electrons into it, since the negative field of the grid tends torepel them; and as the grid approaches equal potential with thefilament, the primary electrons are allowed to increase in velocitysince the repelling negative field of the grid is being reduced. Whenthe grid becomes positive, primary electrons flow into it and thevelocity of these electrons is accelerated by the positive field of thegrid. When sufficient positive grid potential is reached to bring theprimary electrons which strike the grid to a sufliciently high velocity,they eject secondary electrons from the grid.

If these secondary electrons are ejected with sufiicient velocity tocarry them into a region where the field of the positive plate isstronger than the field of the positive grid from which they came, theywill go to the said plate. These secondary electrons which go to theplate thus represent a current leaving the grid, while the primaryelectrons represent a current entering the grid, and the net gridcurrent is the difference between the two currents.

The grid-voltage grid-current curve of a tube without secondary emissionfrom the grid usually obeys a law of the nature of Ig KEg Where Igrepresents the grid current, Eg the grid voltage, K a constant, and a:an exponent which is usually 3/2 to 2.

It is evident that the introduction of a factor which reduces the netgrid current in the proper amount will make the relation more nearlylinear; and I have found that carefully controlled secondary emissionwill do this.

The result of too much secondary emission will be so much reduction inthe net grid current that the curvature reverses. Actually, it ispossible to have the secondary emission factor so high that the netcurrent becomes zero for some positive grid potential, the secondaryelectron current leaving the grid being equal then to the of supply asthe battery and preferably at primary electron-current entering it.

A main factor governing the amount of secondary emission from the gridis the physical material of the grid surface; and it has been found thatthe best material for producing the right amount of secondary emissiondepends largely en the type of tube. For power tubes using 450 volts onthe plate, I have found tantahim to give the best results, tantalumbeing fairly resistantto secondary emission; while for power .tubesusing 250 volts on the plate, a material giving oli secondary electronswith more case, such as molybdenum, gives good results.

Although copious secondary emission may occur from the grid, it ispossible, by separating the plate further therefrom, to make a smallerproportion of the secondary electrons reach the region Where thepositive field of the'plate will prevent them from falling back into thegrid. Consequently the degree of separation of grid from plate to getthe best results is important. although this factor must be so adjustedas to also determine the desired amplification factor and plateimpedance of the tube.

In the drawing, Fig. 1 shows in elevation the novel amplification tubeas a triode; and illustrates the manner of connecting a pair of suchtriodesin the aforesaid patented amplifying circuit. i

In constructing the novel triode, the same is provided with the usualthree elemants' vizz filament 16; grid 11 and plate 12, all beingincluded and mounted in an' evacuated container 13in the usual andwell-known or special manner. The grid 11, however, is of suitablematerial for the particular type of tube, as, for example, tantalumiorthe 450-vo1t type of tube used; or, molybdenum, for a 250-volt type.

A plate 12, or preferably a pair of plates for affordin a strongelectrostatic field surrounding the grid, is located the optimumdistance from said grid 11, giving the desired amplification constant,plate impedance and secondary-emission control in connection with thegrid material used; and a relatively high voltage is arranged to beapplied thereto as from the battery 14, for example, 250-450 volts,while the filament heating-current is received from a suitable sourcepotentials ranging from 2 to 3.5 volts.

in accordance with the use to which these tubes are usually to be put,the same are to be matched to their grid-voltage grid-currentcharacteristics, and two of such tubes 20 and 21 are interconnected inthe manner indicated and more fully set forth in my prior Patent No.1,755,938. In this system and as indicated in Fig. 2, an input circuitrepresented by the leads 22 and 23 has the former lead connected to thegrid of tube 20 and the latter lead to the grid of the tube 21, whilethe filament of said former tube is also connected to the lead 23 andthe filament of the latter tube to the lead 22. The grid currentsflowing are thus compensated to produce distcrtionless transmission ofan incoming signal applied to the said input circuit.

I claim:

1. In a thermionic tube having a cathode, a

grid of material adapted to emit secondary electrons by in'ipact ofelectrons from the cathode, and a plate, the method of obtainingdistortionless amplification by operating the tube on a linear portionof the grid voltage grid-current characteristic, which consists inadjusting the positive potential on the grid to such value that "theemission of secondary electrons therefrom substantially neutralizes thatportion of the pri" mary electrons impinging on the grid from thecathode which causes the non-linear grid-voltgrid-currentcharacteristic.

2. In a thermionic valve, an electron-emitting element, a and anintermediate grid element of tantalum adapted to emit secondaryelectrons, and means to apply a positive potential to the grid of amagnitude to effect an emission of the secondary electrons sufiicient toreduce the grid-current to an extent affording substantially lineargrid-voltage grid-current characteristics.

3. In a thermionic. valve, an electron-emitting element, a plate, and anintermediate grid element or molybdenum. adapted to emit secondaryelectrons, and means to apply a positivepotential to the grid of amagnitude to effect an emission of the secondary electrons sufiicient toreduce the grid-current to an extent affording substantially lineargrid-voltage grid-current characteristics.

LINCOLN THOMPSON.

